As the world's population expands and its economy increases, the increase in the atmospheric concentrations of carbon dioxide is warming the earth causing climate changes. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. Experts say atmospheric levels of carbon dioxide may be double that of the pre-industrial era by the end of the next century, but they also say the levels would be much higher except for a trend toward lower-carbon fuels that has been going on for more than 100 years. Furthermore, fossil fuels cause pollution and are a causative factor in the strategic military struggles between nations. Furthermore, fluctuating energy costs are a source of economic instability worldwide.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The present invention is another product which is essential to shortening that period to a matter of years. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But, with the problems of storage and infrastructure solved (see U.S. application Ser. No. 09/444,810, entitled “A Hydrogen-based Ecosystem” filed on Nov. 22, 1999 for Ovshinsky, et al., which is herein incorporated by reference and U.S. patent application Ser. No. 09/435,497, entitled “High Storage Capacity Alloys Enabling a Hydrogen-based Ecosystem,” filed on Nov. 6, 1999 for Ovshinsky et al., which is herein incorporated by reference), hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
Hydrogen is the “ultimate fuel.” In fact, it is considered to be “THE” fuel for the future. Hydrogen is the most plentiful element in the universe (more than 95%). Hydrogen can provide an inexhaustible, clean source of energy for our planet which can be produced by various processes. Utilizing the inventions of subject assignee, the hydrogen can be stored and transported in solid state form in trucks, trains, boats, barges, etc. (see the '810 and '497 applications).
A fuel cell is an energy-conversion device that directly converts the energy of a supplied fuel into electric energy. Researchers have been actively studying fuel cells to utilize the fuel cell's potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolyte. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In a hydrogen-oxygen alkaline fuel cell, the reaction at the hydrogen electrode occurs between hydrogen fuel and hydroxyl ions (OH−) present in the electrolyte, which react to form water and release electrons:H2+2OH−->2H2O+2e−.At the oxygen electrode, oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−->4OH−.The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
The catalyst in the hydrogen electrode of the alkaline fuel cell has to not only split molecular hydrogen to atomic hydrogen, but also oxidize the atomic hydrogen to release electrons. The overall reaction can be seen as (where M is the catalyst):M+H2->2MH->M+2H++2e−.Thus the hydrogen electrode catalyst must efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional hydrogen electrode material, the dissociated hydrogen atoms are transitional and the hydrogen atoms can easily recombine to form molecular hydrogen if they are not used very quickly in the oxidation reaction. The use of a hydrogen storage alloy as the hydrogen electrode catalyst helps solve these problems by storing the hydrogen atoms in metal hydride form, thereby having a supply of hydrogen readily available for the oxidation reaction. Fuel cells utilizing a hydrogen storage alloy in the hydrogen electrode are disclosed in detail in U.S. Pat. No. 6,447,942 to Ovshinsky et al., the disclosure of which is hereby incorporated by reference.
While the hydrogen electrode may be designed to allow for the storage of hydrogen, the oxygen electrode may also be designed for the storage of oxygen. The oxygen electrodes may include a redox couple material which provides for the storage of oxygen via a valency change. The use of redox couples, for oxygen storage in oxygen electrodes are disclosed in detail in U.S. Pat. No. 6,620,539 to Ovshinsky et al., the disclosure of which is hereby incorporated by reference.
The use of a hydrogen storage alloy in the anode and/or non noble metal oxides at the cathode provides a means to absorb energy, e.g. regenerative braking energy in fuel cells. When the incoming charge reaches the metal hydride electrode, electrolysis of the water in the electrolyte takes place at the hydrogen electrode and the hydrogen generated is absorbed by the hydrogen electrode forming a metal hydride. Similarly the incoming charge oxidizes the catalyst materials at the surface of the oxygen electrode to their corresponding oxides. The presence of metal hydride at the hydrogen electrode and/or the metal oxide at the oxygen electrode begins to act like a battery with a finite amount of energy being stored. When the energy supply is ceased, the energy stored in the “battery” can be used to do useful work. Since the operating potential of this power source can be higher than that of the typical alkaline fuel cell, itself, the overall fuel cell efficiency will go up until all the charge from the power source is exhausted. FIG. 1 shows the increase in electrode potential of an alkaline fuel cell upon receiving a pulse of electrical current. FIG. 2 shows the ability of the fuel cell to accommodate pulses of electrical current having different magnitudes.
The length of time for which the higher operating potential lasts depends on the amount of active materials present and the efficiency at which the charge is accepted. In this case it is the amount of metal hydride and metal oxide catalyst present. To increase this time, one can resort to increasing the catalyst loading. However increasing the catalyst loading will have negative consequences to the operation of a normal fuel cell. Therefore, it is highly desirable to obtain alternatives for providing a prolonged higher operating potential in a fuel cell.